Integrated circuit (IC) packages such as Flip Chip (FC) and Ball Grid Array (BGA) packages are directly attached to printed circuit boards (PCB) by means of surface mount solder joints. This technology is widely used today in electronic items from computers to cellular telephones to produce very efficient, high-density products.
However, when these packages are mounted on printed circuit boards, mechanical stresses transmitted to the solder joints can cause the interconnect system to fail by any number of mechanisms, such as cracking of the solder joint or circuit trace, or a solder pad may delaminate from the PCB. These mechanical stresses can arise from a number of sources, such as thermal mismatch between the package and the PCB, but most commonly are caused by mechanical shock when the electronic product is dropped. One common solution to this problem has been known for some time. Specifically, a liquid polymerizable material, called an “underfill”, is flowed under the package after it is soldered to the PCB. Once the underfill has completely filled the small gap that exists between the bottom of the flip chip and the substrate, the material is hardened by polymerization. The hardened, polymerized underfill locks the IC package and the PCB together so that there is little if any differential movement. By controlling excessive stresses that would otherwise form in the joints between the chip and PCB, a reliable assembly can be fabricated.
While the conventional use of underfill has solved the mechanical shock problem on PCBs, it has given rise to a series of significant manufacturing problems. First, the prepolymerized liquid underfill must be applied as a secondary process with special equipment. Typically, the underfill is applied to one, two or three edges of the assembled package and allowed to flow all the way under the mounted package. Once the material has flowed to opposite edges and all air has been displaced from under the chip, additional underfill may be dispensed to those outer edges to form a fillet. The fillet increases reliability and is generally preferred even though it requires additional manufacturing time. Next the assembly is baked in an oven to polymerize and harden the underfill, again adding time to the process. This baking process normally takes 10 to 30 minutes, although it can take several hours, and the added equipment and its maintenance adds significantly to manufacturing costs. Thus, while the use of underfill helps to alleviate the mechanical shock problem and provides a commercial solution, the electronic device manufacturing industry seeks more efficient manufacturing methods with lower associated costs.
Recently, advances have been made which improve and streamline the underfill process. One method involves dispensing underfill onto the IC package prior to assembling to the board. An underfill that is pre-dispensed before soldering should contain flux or have properties that facilitate solder joint formation. Since the pads on circuit often oxidize and since tin/lead solder bumps on IC packages are usually oxidized, the flux and underfill must be designed to reduce these oxide coatings and facilitate solder joint formation. While simplifying the IC package assembly process, this method still requires extra steps. In spite of the numerous advantages provided by IC package underfill technology, a simpler method of applying underfill is needed. A need also exists for process methodologies which reduce the number of process steps required with underfill application.